The proliferation of battery-operated portable electronic devices has resulted in an increased demand for improvements in the power efficiency of such devices. Many such devices now incorporate digital components that are capable of transitioning into and out of a variety of reduced power consumption states. Some computer systems, for example, may be capable of transitioning to a standby mode, wherein the processor is halted, but power is still provided to both the processor and the memory subsystem. Other systems may implement a sleep state, wherein the processor and other system components are powered down, but memory remains powered, either at full power or at a reduced power level. Still other systems may implement a hibernate state wherein the contents of the memory subsystem are saved to disk and the entire system is powered down. The saved contents of the memory subsystem may be used to restore the state of the system when the system is brought out of a hibernation state.
Many of the systems that are capable of multiple power states incorporate power control logic, sometimes in the form of a power controller state machine. The granularity of the control provided by such logic, however is limited to powering entire subsystems up or down, or reducing the power consumption of such subsystems by gating off a clock signal or by operating the subsystems at a reduced voltage (if the circuitry is designed for such low-voltage operation). Further, the transitions between power states are controlled by the power control logic, with no direct power control of individual subsystems available for access and control by software executing on a processor within the system.